Enhanced electrocatalytic activity of fluorine doped tin oxide (FTO) by trimetallic spinel ZnMnFeO4/CoMnFeO4 nanoparticles as a hydrazine electrochemical sensor

In the present study, ZnMnFeO4 and CoMnFeO4 tri-metallic spinel oxide nanoparticles (NPs) were provided using hydrothermal methods. The nanoparticles have been characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and electrochemical techniques. A reliable and reproducible electrochemical sensor based on ZnMnFeO4/CoMnFeO4/FTO was fabricated for rapid detection and highly sensitive determination of hydrazine by the DPV technique. It is observed that the modified electrode causes a sharp growth in the oxidation peak current and a decrease in the potential for oxidation, contrary to the bare electrode. The cyclic voltammetry technique showed that there is high electrocatalytic activity and excellent sensitivity in the suggested sensor for hydrazine oxidation. Under optimal experimental conditions, the DPV method was used for constructing the calibration curve, and a linear range of 1.23 × 10−6 M to 1.8 × 10−4 M with a limit of detection of 0.82 ± 0.09 μM was obtained. The obtained results indicate that ZnMnFeO4/CoMnFeO4/FTO nano sensors exhibit pleasant stability, reproducibility, and repeatability in hydrazine measurements. In addition, the suggested sensor was efficiently employed to ascertain the hydrazine in diverse samples of cigarette tobacco.

and ZnMnFeO 4 nanoparticles. The process of hydrothermal synthesis of ZnMnFeO 4 nanoparticles, which was carried out significantly in this study, was conducted similarly to the synthesis process of CoMnFeO 4 12,37 . In 50 mL of distilled water, a particular amount of Mn(NO 3 ) 2 ·4H 2 O (2 mM), Fe(NO 3 ) 3 ·9H 2 O (4 mM), and Zn(NO 3 ) 2 ·6H 2 O (6 mM) were dissolved and stirred by a magnetic stirrer for 30 min to obtain a homogenous solution. Following that, the pH was adjusted to 12.0 by adding the ammonia aqueous solution gradually and dropwise to the solution under stirring, and then the cations' hydroxides were coprecipitated. Next, the obtained mixture was transmitted into a 100-mL Teflon™-lined autoclave and heated at 180 °C for 24 h. After the final product reached ambient temperature, it was separated by a magnet, centrifuged, and washed with distilled water and ethanol several times until a neutral pH was achieved. Then, the product was placed at 70° inside the oven for 12 h and dried completely. Thereafter, the assynthesized powder was calcined at 600 °C for 3 h.
Fabrication of modified electrode. In the first phase, it's extremely important to prepare and activate the substrate surface before modifying the electrode. As a consequence, FTO glass was degreased and cleaned by ultrasonication for 30 min, which was implemented in 3 phases, including cleaning in distilled water, acetone, and ethanol, respectively. The time spent on each phase is 10 min. At last, it was allowed to be dried under a flow of nitrogen gas immediately before use. Then, 1.5 mg of synthesized CoMnFeO 4 nanoparticles were dispersed in 1 mL of N-methyl pyrrolidone via an ultrasonic bath for 10 min. This suspension (10 µL volume) was drop-cast on a 1 cm × 1 cm FTO glass substrate and dried in an oven at 80 °C for 1 h. Next, it was stabilized and annealed in air at 400 °C for 3 h. In the end, a light tan film was obtained on the FTO substrates. Following this, the same phases were carried out to deposit ZnMnFeO 4 on the CoMnFeO 4 electrode. After all, the modified electrode, which is indicated as ZnMnFeO 4 /CoMnFeO 4 /FTO was rinsed with deionized water.

Results and discussion
Characterizations of the morphology and structure. The spinel ferrites are used in electrochemical sensors because of some features, including their superlative electrical and photoelectrochemical performance, high chemical stability, magnetic properties, low price, and good conductivity. Briefly, the modifier used in the present study causes HZ response enhancement and oxidation potential reduction considerably, which is due to its exceptional conductivity, high adsorption, and high electrochemical surface area of ZnMnFeO 4 /CoMnFeO 4 / FTO. By means of the FT-IR spectra presented in Fig. 1 38 . In the FT-IR spectrum of CoMnFeO 4 (Fig. 1b), the absorption band emerges at 589 cm −1 as a result of the intrinsic stretching vibrations of metal ions (Co-O and Fe-O) bonding the tetrahedral site 12 .
The absorption peak that can be seen at 1630 and 3440 cm −1 corresponds to water molecules 39 . The CoMnFeO 4 and ZnMnFeO 4 ferrite samples are synthesized by the hydrothermal route, and in Fig. 1c,d, the patterns of X-ray diffraction for these samples can be clearly seen. For both samples, which have proper crystallinity and welldefined diffraction lines, a single-phase spinel structure was perceived without any unfavorable phase. There is no obvious peak detected for both samples. This bears witness to the high purity of the synthesized nanostructures. In Fig. 1c, the Apparent diffraction peaks for synthesized ZnMnFeO 4 nanoparticles at planes of (111), (220), (311), (400), (422), (511), and (440) adapted well to the standard pattern reported in JCPDS card no. 01-074-2400. And in Fig. 1d, the relevant peaks of CoMnFeO 4 at planes of (220), (311), (222), (400), (422), (511), and (440) in the XRD pattern could be indexed to the standard pattern reported in JCPDS card no. 00-001-1121. Based on the XRD diffractograms of both nanoparticles, it can be seen that every one of the peaks is either all even or all odd. This indicates that the samples are spinel in phase 40 . The better electrochemical sensor response is predominantly dependent on the electrode surface area, which was utilized in modified electrodes, and the size and morphology of the electrocatalysts. The purpose of the FE-SEM implementation was to investigate the morphological features and particle sizes of ZnMnFeO 4 and CoMnFeO 4 NPs. The synthesized ZnMnFeO 4 sample's SEM image is shown in Fig. 2a,b. In conformity with this delineation, the size of nanoparticles in these nanostructures tends to be smaller than CoMnFeO 4 , less than about 35 nm in diameter. The image of CoMnFeO4 nanoparticles less than 45 nm in size can be seen in  www.nature.com/scientificreports/ and CoMnFeO 4 NPs electrode surface elements was designated with X-ray energy scattering spectroscopy. Both EDX spectra revealed a peak at 0.51 keV for O Kα. The revealed peak is because of the oxygen atoms in ZnMnFeO 4 and CoMnFeO 4 nanoparticles. There are three exclusive peaks for the Zn, Mn, and Fe elements in the ZnMnFeO 4 graph (Fig. 2e) and also for the Co, Mn, and Fe elements in the CoMnFeO 4 graph (Fig. 2f). The presence of desired elements was confirmed by the findings in the prepared compositions, which are uniformly distributed without the appearance of any impurities. Given that, it leads to the conclusion that the synthesis of ZnMnFeO 4 and CoMnFeO 4 has been efficiently accomplished. As shown in Fig. 2g 6 ] −4 as a redox probe at a scan rate of 50 mV s −1 , were evaluated by using the cyclic voltammetry technique significantly and were compared to each other (Fig. 3a). According to the results, the modification of electrodes with ZnMnFeO 4 /CoMnFeO 4 created a reduction in the peak-to-peak separation and had higher peak currents compared to the others. In fact, the features of the modified electrode, such as higher electrical conductivity and the large electroactive surface area, make these significant differences between the bare electrode and the ZnMnFeO 4 /CoMnFeO 4 /FTO electrode. The electrical conductivity and electron transition resistance of the modified electrode surface have been checked by electrochemical impedance spectroscopy. The EIS spectrum record of the bare FTO electrode and the three modified electrodes cited above is shown in Fig. 3b. Studying the process of charge transfer on the surface of the modified electrode is a practical way to do so. The reason is that there is a double-layer capacitance and also a resistance to interfacial charge transfer after the modification on the electrode surface. The EIS Nyquist spectra involve a semicircular region and a linear region. The semicircular diameter in the high-frequency region indicates the resistance of the interface charge transfer (Rct) that is shown in Fig. 3b. The Warburg element represents the diffusion process and is associated with the low-frequency region linear section. The transition resistance of the electrodes is characterized by employing a semicircular diameter in the Nyquist designs.
In Fig. 3b, it can be observed that the resistance to charge transfer for the modified CoMnFeO 4 /FTO electrode is less than that of the bare electrode. It undoubtedly shows that the CoMnFeO 4 nanoparticles act as a stimulus and speed up the interfacial charge transfer. The diameter of the semicircle in the curve corresponding to the ZnMnFeO 4 /FTO electrode is decreased in proportion to the CoMnFeO 4 /FTO electrode, which represents the high electrical conductivity of the ZnMnFeO 4 compared to the CoMnFeO 4 NPs. In the curve related to ZnMnFeO 4 /CoMnFeO 4 /FTO, a decrease in charge transfer resistance can be seen after coating the ZnMnFeO 4 on the CoMnFeO 4 /FTO, which is a result of a higher electrochemical active surface area and the enhanced charge transfer rate of ZnMnFeO 4 /CoMnFeO 4 /FTO compared to the other modified electrodes. The Warburg element is also seen in the impedance spectrum (Fig. 3b), indicating the electrolyte diffusion into the coating and showing the porosity of the modifier. This is an essential and critical parameter in the properties of catalysts. It indicates that these two NPs together are being used successfully and have a positive effect on each other. www.nature.com/scientificreports/ The electrochemically active surface area of the modified electrode was studied, and results were reported in the supplementary file (Fig. S1).

Electrochemical behavior of hydrazine on ZnMnFeO 4 /CoMnFeO 4 /FTO. The cyclic voltammetry
technique was conducted to assess the electrochemical behavior of hydrazine on manifold-modified electrodes. The cyclic voltammetric response of 0.1 mM hydrazine in the 0.1 M ammonia buffer (pH = 9.0) was recorded in the potential range of − 0.1 to 0.85 V. As reflected in Fig. 4a, the oxidation of hydrazine on a bare FTO substrate demands a high positive potential (0.575 V), which shows a considerably lower peak current (curve a). In the b-curve, the hydrazine oxidation is shifted towards a less positive potential (0.423 V) by modifying the electrode surface with CoMnFeO 4 nanoparticles, and the current is increased relative to the bare electrode state (22.0 μA). On the other side, in the c-curve that corresponds to the ZnMnFeO 4 /FTO electrode, the hydrazine oxidation potential is moderately shifted towards the positive potential (0.431 V) in comparison with the CoMnFeO 4 / FTO electrode, but the current is increased to about 29.0 μA. It does indicate that the electrodes modified with CoMnFeO 4 NPs and ZnMnFeO 4 NPs have catalytic properties on hydrazine. In the d-curve, it can be seen that when the electrode is modified with ZnMnFeO 4 /CoMnFeO 4 NPs, there is a shift towards less positive potentials in the oxidation potential of hydrazine and also a sharp peak at potentials less than 0.4 V, about 0.34 V, and the current is increased to about 39.0 μA.
Given that, the ZnMnFeO 4 /CoMnFeO 4 /FTO has a better electrocatalytic property for hydrazine when compared with the other modified electrodes. A stable current was achieved in less than 5 s by adopting a modified electrode. It indicates a rapid electron exchange on the modified electrode surface and satisfactory catalytic performance.

Effects of buffer type and pH value.
To verify the effect of the electrolyte solution pH value, the 0.1 M phosphate buffer was used in the pH range of 5.0-9.0, including 0.1 mM hydrazine, reaching the greatest current response and the best oxidation potential of the sensor for hydrazine. In Fig. 4b, it is apparent that the peak potential and peak current of the ZnMnFeO 4 /CoMnFeO 4 /FTO electrode are highly dependent on the pH of the solution. This is due to the shift of hydrazine oxidation peak potentials to negative potentials with increasing pH of the solution, based on the following Eq. (1): The slope of − 61.3 mV was obtained from the potential of the E p -pH diagram, which is close to the theoretical Nernst value (Fig. 4c). Accordingly, it indicates that equal numbers of electrons and protons were engaged in the oxidation reaction of hydrazine (Eq. 2).
The pH value of the supporting electrolyte is a significant parameter for the effective electrocatalytic behavior of hydrazine, and as shown in Fig. 4d, the response of hydrazine increased along with an increasing pH value. It has been suggested that the enhancement of the current response in alkaline solutions is caused by the adsorption of hydrazine to the electrode surface. In this study, pH = 9.0 was chosen as the eligible pH, and indeed, the effect of buffer type on the electrooxidation of hydrazine was studied at the same pH value. Consequently, diverse buffers, namely ammonia, phosphate, and Britton-Robinson with a concentration of 0.1 M, were used to accomplish this. Figure 4e shows that the voltammetric response of the ammonia buffer is superior to that of buffers. The reason is that the hydrazine oxidation peak emerged at the lower potential in contradiction to other buffers and increased the current.
Effect of scan rate. To investigate the kinetic reaction of the hydrazine oxidation and its electron transfer mechanism, the cyclic voltammetry technique was used at 5-300 mV s −1 scan rates by ZnMnFeO 4 /CoMnFeO 4 / FTO electrodes in 0.1 M ammonia buffer (pH 9.0) containing 0.1 mM hydrazine (Fig. 4f). According to the results, the peak current increases progressively as the scan rate increases. The diagrams of peak currents (I p ) versus scan rate (ν) and second root of scan rate (ν 1/2) were plotted to perceive and interpret the diffusion or absorption nature of the electrode process. Considering that the peak currents are linearly proportional to the square root of the scan rate, the electrocatalytic oxidation of hydrazine on the ZnMnFeO 4 /CoMnFeO 4 /FTO was controlled by a diffusion-controlled process (Fig. 4g,h). As the scan rate increases, the peak potential of hydrazine electrooxidation shifts towards a positive potential. In other words, there are kinetic constraints at high scan rates 41 .
A Tafel plot was depicted at the scan rate of 5 mV s −1 (Fig. 4i) for further studying the kinetic parameters (α). According to Tafel Eq. (3): where α is the transfer coefficient, T is the temperature (K), F is the Faraday constant (96,485 C mol −1 ), R is the universal gas constant (8.314 J K −1 mol −1 ) and n a refers to the number of transferred electrons in determining step, The Tafel plot slope is 6.2156 mV decade −1 . On this account, by substituting these values in Eq. (3), the α parameter is obtained at 0.77 assuming n a equals 1. The relationship between peak potential (E p ) and the natural logarithm of scan rate (log ν) can be defined by the Laviron equation.
According to this equation and also by using the slope value of the E pa vs. log ν diagram (Fig. 4j) and the α value that was attained to be 0.77 from Eq. (3), the number of electrons engaged in the rate-limiting step of hydrazine (n) was estimated to be 1.  www.nature.com/scientificreports/ Additionally, a linear relationship between the E pa and I pa was proposed by depicting the diagram of log E pa versus log I pa (Fig. 4k) in the scan rates work. Accordingly, the regression equation was: log E pa (V) = 0.6347 log I pa (μA) − 1.5122 (R 2 = 0.9693). The hydrazine oxidation, however, becomes more challenging in high-rate scanning because, by increasing the scan rate, a considerable change occurs with expanding peak currents in the hydrazine oxidation potential displacement towards more anodic potentials.
Based on all previous findings and results, the hydrazine electrochemical oxidation mechanism has been reported to be conducted via a 4-electron process and results in the production of nitrogen gasses by the following equations (Eq. 6) 18,42 .
The present survey found that the hydrazine oxidation process is an irreversible oxidation process in which hydrazine oxidizes to produce N 2 and hydronium ions (H 3 O + ) along with four electrons. In this part, the ratelimiting step is the first step containing one-electron transfer (Eq. 4), followed by the fast second step containing the three-electron transfer process (Eq. 5) 34,42 : Chronoamperometric studies. The chronoamperometry technique was performed in a pH 9.0 ammonia buffer (0.1 M) containing different concentrations of hydrazine (from 15 to 75 µmol L −1 ) to ascertain the diffusion coefficient of the hydrazine. The potential of the working electrode was set at 0.4 V, and the Cottrell equation was applied (Eq. 7) to define the current response for an electroactive compound that is controlled by a diffusion mechanism: C and D are the bulk concentration (mol cm −3 ) and diffusion coefficient (cm 2 s −1 ), respectively; A is the active area of the modified electrode that was obtained to be 1.72 cm 2 , and I refers to the diffusion current of hydrazine from the bulk solution to the interface of the solution/electrode, and the other signs have their specific conventional meanings. The experimental diagrams that have been plotted for the several hydrazine concentrations are viewed in Fig. 5a. The diagram of I versus t −1/2 can be observed in Fig. 5b. For hydrazine, the average diffusion coefficient was computed by plotting the slope values versus different hydrazine concentrations (Fig. 5c); D = 4.29 × 10 −6 (cm 2 s −1 ).  Figure 6a shows that as the concentration of hydrazine increases, the electrocatalytic response gets sharper. A linear calibration plot was also attained between the hydrazine concentrations and associated peak current (I p ) as I (μA) = 0.347 [hydrazine] (μA/mM) + 0.021; R 2 = 0.997. The linear range was achieved at the concentration range of 1.2-184.7 µM. Sensitivity, LOD, and LOQ are estimated as 0.347 μA mM −1 , 0.82 μM and 2.75 μM for hydrazine, respectively, through the slope of the calibration plot (Fig. 6b).
ZnMnFeO 4 /CoMnFeO 4 as a modifier provides an acceptable ambiance for hydrazine indication since it has a high electroactive surface area. It can be noted that the high electron communication features are caused by the high sensitivity of this sensor, which enhances the direct charge transfer between the active area of the modifier and the FTO substrate. The analytical performance of the suggested sensor that has been used to detect hydrazine compared to the electrodes reported earlier is shown in Table 1.
The detection limit of the suggested sensor is better or at least similar to other modifiers of electrodes that have been previously stated in the table. The findings demonstrate that the sensor was adequate and qualified for hydrazine detection.   www.nature.com/scientificreports/ maximum concentration of the foreign substances, the limit of tolerance was taken, which produced an approximate error in the analytical response of the analyte of less than ± 5% (Fig. S2). As can be observed in Table 2, the small change in the DPV response of hydrazine is caused by the eightfold ascorbic acid and uric acid, tenfold citric acid, 300-fold glucose, Lactose and Fructose, and 500-fold ethanol, K + , Na + , Cu 2+ , Ni 2+ , Br − , Cl − , SO 4 2− , (CH 3 CO 2 ) − , and NO 3 − . According to the findings, the suggested sensor has been chosen properly. Therefore, it can be applied to detect hydrazine in the presence of biological molecules and environmental pollutants.
Real samples. For real samples of hydrazine, selective detection was provided by the suggested sensor in the present study, and it has been shown whether this method is applicable. The hydrazine and derivatives that concurrently exist in tobacco products. To assess the amount of hydrazine, however, the diluted tobacco solution was injected directly into the electrolyte solution. Next, the amount of hydrazine standard solution that had been assessed earlier was added, and then, to detect the concentration of hydrazine, the standard addition method by differential pulse voltammetry (DPV) technique was accurately implemented. In Fig. S3, it can be observed the test of common hydrazine in real samples for three brands of cigarettes (C 1 -C 3 ) using ZnMnFeO 4 /CoMnFeO 4 / FTO. Considering that the findings of the calculated recovery were in the range of 97.77-100.71% with a R.S.D. (for three repetitions) of below 3%, one can deduce that the suggested sensor has a desirable selectivity ( Table 3).
The stability, repeatability, and reproducibility of ZnMnFeO 4 /CoMnFeO 4 /FTO. The electrode was kept in the air for one month to analyze the modified electrode's stability. Afterward, required assessments were conducted on the 7th and 30th days. Based on the findings of this study, it can be declared that the modified electrode has 97.25% of its initial current response after 7 days and 95.88% for a month, which has desirable stability in view of its high specific surface area (Fig. S4a). Employing five consecutive tests with the same modified electrode and applying cyclic voltammetry, the modified electrode response repeatability in hydrazine measurement was precisely measured. The R.S.D. of peak currents was estimated at 1.78%, which demonstrates satisfactory and adequate repeatability for the modified electrode (Fig. S4b). In addition, four different electrodes were modified with ZnMnFeO 4 /CoMnFeO 4 for studying the reproducibility of the sensor, the CVs were registered, and the R.S.D. was calculated at 2.05% (Fig. S4c). The findings conclusively reveal that ZnMnFeO 4 /CoMnFeO 4 / FTO has good stability as a sensor and also possesses adequate quality of repeatability and reproducibility in hydrazine measurement.
The sensor's low reusability is one of the work's limitations. After four washes, the modified electrode's CV response recovered as much as 98.4%, 94.6%, 91%, and 82.9% of the original response signal (Fig. S4d).

Conclusion
In this study, a new assay for the oxidation of hydrazine was prepared based on the modified FTO electrode. The modified FTO electrode was prepared as ZnMnFeO 4 /CoMnFeO 4 /FTO through deposition of the ZnMnFeO 4 NPs on the surface of the CoMnFeO 4 NPs. The results showed that this modified electrode provides significantly enhanced electrolytic activity with a remarkable decrease in overvoltage and provides better peak current intensity when compared with bare FTO electrodes. To detect hydrazine using the DPV technique, the modified electrode showed high sensitivity and selectivity and a low detection limit. Furthermore, there are several remarkable advantages to the suggested sensor, including long-term stability and repeatability, simple preparation, and low cost.
The interference from current co-existing chemical species, which are present in excess concentration, was tolerated by the examined electrode. In the final analysis, the modified electrode was employed to detect the amount of hydrazine in cigarette samples, and the findings were satisfactory. www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.